Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

86% (01:39) correct
13% (01:06) wrong based on 198 sessions

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

* 16 * 20 * 25 * 32 * 35

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

C. 25

NOPE..

so it takes 60 years..so from 1979--60 years is 2039..so when we see it next will be 2039-2004 or 35 yrs from now..

Thanks a lot. +1. We'll edit the wording of this one.

fresinha12 wrote:

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

* 16 * 20 * 25 * 32 * 35

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

So... I get that 2039 - 1979 equals 35 but how do you recognize the multiplication of 3 * 2 * 2* 5 to give you the years.

I did separate the 12 and 20 yrs in Greatest Common Factor and Least Common Multiple (4)*(15) but I don´t understand why the multiplication of both will give me the answer of when both comets will see each other again.

Hi, the corrent answer is E ie after 35 years. Comet A is seen after 12 years and B after 20 years. So they will be seen together after LCM(12,20) = 60 years. After every 60 years they will be seen together. They were seen together in 1979, so 60 years after 1979 ie 2039. So number of years 2039-2004(2004 is current year) = 35 years.

For people who dont know what LCM stands for it is Least Common Multiple. LCM of two numbers can be found out as below: 12 = 2*2*3 (Find the prime factors) 20 = 2*2*5 (Find the prime factors) LCM(12, 20) = Common * Uncommon = 2*2*3*5 = 60

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

Comet " B " will appear once in 20 years... 20 * 1 = 20th year 20 * 2 = 40th year 20 * 3 = 60th year

Comet " A " will appear once in 12 years.. 12 * 1 = 12th year 12 * 2 = 24th year 12 * 3 = 36th year 12 * 4 = 48th year 12 * 5 = 60th year

When we compare above table, Both the comet will appear again in 60th year from 1979 = 1979+60 =2039th year current year is 2004 therefore 2039-2004 = 35 years..

LCM of 12 and 20 is 60 hence they would meet in 2029.i.e. 2029-2004 = 35

Why LCM......Lcm can also be explained as occurrence of different digits after some interval so that the result is same.
_________________

I will give a Fight till the End

"To dream anything that you want to dream, that is the beauty of the human mind. To do anything that you want to do, that is the strength of the human will. To trust yourself, to test your limits, that is the courage to succeed." - Bernard Edmonds

A person who is afraid of Failure can never succeed -- Amneet Padda

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

LCM of 12 and 20 is 60. Hence 60 years after 1979 these will be seen again. Now it is 2004. So it is alrady 25 years since 1979. So additional 60-25 = 35 Years will be needed. Hence "E"

Hi, the corrent answer is E ie after 35 years. Comet A is seen after 12 years and B after 20 years. So they will be seen together after LCM(12,20) = 60 years. After every 60 years they will be seen together. They were seen together in 1979, so 60 years after 1979 ie 2039. So number of years 2039-2004(2004 is current year) = 35 years.

For people who dont know what LCM stands for it is Least Common Multiple. LCM of two numbers can be found out as below: 12 = 2*2*3 (Find the prime factors) 20 = 2*2*5 (Find the prime factors) LCM(12, 20) = Common * Uncommon = 2*2*3*5 = 60

Send me kudos if you like my post.

Kudos.

This is how I calculated mine although I don't think I would be able to explain it as well as you.

Hi, the corrent answer is E ie after 35 years. Comet A is seen after 12 years and B after 20 years. So they will be seen together after LCM(12,20) = 60 years. After every 60 years they will be seen together. They were seen together in 1979, so 60 years after 1979 ie 2039. So number of years 2039-2004(2004 is current year) = 35 years.

For people who dont know what LCM stands for it is Least Common Multiple. LCM of two numbers can be found out as below: 12 = 2*2*3 (Find the prime factors) 20 = 2*2*5 (Find the prime factors) LCM(12, 20) = Common * Uncommon = 2*2*3*5 = 60

Send me kudos if you like my post.

I got E as well. But, it took a while to arrive at the answer.

I know LCM, but didn't know that we can use it for this problem. Can you please explain any other problem scnerios where LCM can used to solve a problem.

Waiting for the day when I will be able to approach and solve maths the way you guys do it! truly commendable!

Both comets have different frequencies i.e. 12 yrs and 20 yrs. If B covers 20 yrs A will be covering 12 yrs At the end of B's 20 yrs , A's 24 yrs .... ....... in 60 yrs they will be visible together !!!

OR

LCM 12, 20 = 60 MEANS that both are visible together at every 60 years. In 1979 both appeared together, so 60 yrs after again they will be visible together, i.e in 2039. Now it is 2004 so in another 35 years it will be seen together again.

(E) Wins
_________________

" Make more efforts " Press Kudos if you liked my post